380 



Mr. A. Mallock 



[May 27, 



suspended to receive any mercury which might flow from the cistern. 

 The matter to be tested was placed inside the cylinder, which was 

 then completely filled with water, the cover was fitted on, and the 

 whole placed in a strong steel explosion vessel also full of water, and 

 the pressure was then pumped up as required. (Fig. 1.) 



Thus there was practically no difference of pressure inside and 

 outside the glass cylinder, and the volume of mercury which flowed 

 through the tube (ascertained by weighing the 

 contents of the cup) gave a measure of the 

 volume compression of the contents of the 

 former. 



The only correction required is that for the 

 volume compression of the glass and water, 

 that for the small volume of mercury employed 

 being negligible. The same method could 

 probably be used with advantage at even the 

 highest attainable pressures. 



To sum up what is known as to the be- 

 haviour of matter when the volume pressure 

 varies, it may be said that for any small change 

 the variation of volume is directly proportional 

 to the variations of pressure, and that while iu 

 the case of gases there is no limit to the 

 possible expansion, no force, however large, 

 can reduce the volume to less than the sum of 

 the volumes of the molecules. 



With liquids and solids there is a limit to 

 the possible expansion, which is reached when 

 the negative pressure exceeds the inter-mole- 

 cular cohesion. When this limit is passed the 

 extra space is either filled with the vapour of 

 the substance, or is vacuous, and if the sub- 

 stance is a solid fracture occurs. 

 Yig. l. For a substance which has the same pro- 



perties in all directions volume compression 

 cannot cause rupture, and a cube, for instance, would remain a cube, 

 no matter how much it was compressed, the only change being in 

 the absolute dimension. 



For crystals or other substances which have different compressi- 

 bilities in different directions this is not so obvious, and it is not 

 known whether the different elastic properties of crystals in the 

 principal axes depend on compressibility or on rigidity, or both. 



Rigidity, or resistance to change of shape, has been more fully 

 investigated, experimentally at any rate, than compressibility. 



What happens during distortion can be illustrated by supposing 

 that two opposite faces of a cube are fixed to two parallel planes, and 

 that these are forced to slide while remaining parallel and at a 



